Recently, an ionomer resin has been used as a base resin for a cover of golf balls (e.g. Japanese Laid-Open Patent Publication No. 49(1974)-49727). Particularly, the ionomer resin is exclusively used in a two-piece golf ball containing a solid core therein.
This is because the ionomer resin is superior in durability, cut resistance and impact resilience and, further, it is easily processed.
However, the ionomer resin is inferior in hit feeling and control properties (ease of putting spin on the ball) in comparison with a balata used as the base resin for the cover of the thread wound golf ball, since the ionomer resin has high hardness and stiffness.
In order to improve hit feeling and control properties, Japanese Laid-Open Patent Publication No. 1(1989)-308577 suggests to soften a high-rigid ionomer resin by blending or mixing a soft ionomer resin comprising a sodium or zinc salt of a terpolymer of an olefin, an unsaturated monocarboxylic acid having 3 to 8 carbon atoms and an unsaturated acrylate having 2 to 22 carbon atoms therewith.
However, even if the soft ionomer resin as described above is blended, golf balls having hit feeling similar to that of the golf ball with the balata cover are not obtained.
The balata cover is characterized by the fact that stiffness modulus is comparatively high in spite of low hardness, which enables the balata-covered golf ball to have excellent control properties and hit feeling.
However, in the method of blending the soft ionomer resin described in Japanese Laid-Open Patent Publication No. 1(1989)-308577, when the trial of obtaining the hardness similar to that of the balata cover is made, the stiffness modulus becomes too small in comparison with the balata cover. On the other hand, when the trial of obtaining the stiffness modulus similar to that of the balata cover is made, the hardness becomes too large in comparison with the balata cover. In both cases, the hit feeling of the resulting golf ball is quite different from that of the balata-covered golf ball.
In addition to the above suggestion, various efforts of softening the ionomer resin to improve the hit feeling and the control properties have been made. However, satisfactory results have not been obtained.
As described above, the ionomer-covered golf ball is inferior in hit feeling and control properties in comparison with the balata-covered golf ball because the ionomer resin has high hardness and is rigid. And softening the ionomer resin to improve hit feeling and control properties has not been effective.